Thickening or gelling agents, used in electrochemical cells to gel liquid electrolytes in order to reduce electrolyte leakage, until recently, have mainly been of natural products such as starch, cellulose or their derivatives. However, on standing or during cell discharge, a large percentage of liquid separates from many of such thickened or gelled electrolytes. Further difficulties have been encountered in uniformly gelling large quantities of electrolyte and, thereafter, dispensing the gelled electrolyte into the cell. Such lack of uniformity, which generally worsens on standing as the liquid separates from the gel, can result in the inaccurate addition of electrolyte to the cell with resultant inconsistent cell properties. The high viscosity of such thickened or gelled electrolyte additionally increases the difficulty in pumping and accurately dispensing proper amounts of electrolyte into a cell.
The use of larger quantities of gelling agent may alleviate some of the separation problems but at the cost of displacing some of the active material in the cell. Furthermore, the greater degree of gelling of the electrolyte also generally decreases the ionic conductivity of the electrolyte which results in high internal cell resistance as well as increasing problems inherent with the already high viscosity of the gels.
In some cells instead of the electrolyte alone being gelled, a mixture of the electrolyte with either the anodic or cathodic material is gelled. However, the generally particulate anodic or cathodic materials tend to settle out from the gel or are clumped together by the gelling agent, resulting in lack of uniformity and similar inaccurate addition of anodic or cathodic materials to the cell. Further, the nonuniform gel results in uneven and inefficient consumption of the anodic or cathodic materials during cell discharge.
Materials which have been found to be capable of more uniformly gelling a liquid or a mixture of a liquid and particulate matter have, in some instances, not been stable in the strong alkali electrolyte used in many aqueous cells, particularly at the high temperatures and after the long storage periods to which such cells are subjected. Further, such materials are wholly or partially petroleum based, in contrast to the previously used gelling agents, which were based on vegetable material. With the present high price of petroleum, such petroleum based gelling agents have become very costly. Additionally, some of the monomers or intermediates used in the manufacture of the petroleum based gelling agents may adversely affect the environment.